rhgb is the red hat graphical boot – This is a GUI mode booting screen with most of the information hidden while the user sees a rotating activity icon spining and brief information as to what the computer is doing.

quiet hides the majority of boot messages before rhgb starts. These are supposed to make the common user more comfortable. They get alarmed about seeing the kernel and initialising messages, so they hide them for their comfort.

I personally prefer a faster bootloader wait time, hence I lowered my GRUB_TIMEOUT from 5 seconds to 1 seconds.

This is my /etc/default/grub output after I removed both rhgb and quiet option: